1) Field
Embodiments of the present invention generally relate to plasma processing equipment, and more particularly to methods of controlling temperatures during processing of a workpiece with a plasma processing chamber.
2) Description of Related Art
In a plasma processing chamber, such as a plasma etch or plasma deposition chamber, the temperature of a chamber component is often an important parameter to control during a process. For example, a temperature of a substrate holder, commonly called a chuck or pedestal, may be controlled to heat/cool a workpiece to various controlled temperatures during the process recipe (e.g., to control an etch rate). Similarly, a temperature of a showerhead/upper electrode or other component may also be controlled during the process recipe to influence the processing. Conventionally, a heat sink and/or heat source is coupled to the processing chamber to control the temperature of a chamber component at a setpoint temperature. Typically, a first controller, such as a PID (proportional-integral-differential) controller is employed for feedback control of the heat transfer between the temperature-controlled component and a heat sink while a second controller is employed for feedback control of the heat transfer between the temperature-controlled component and a heat source. Each of the first and second controllers generally operate in isolation of the other, independently executing their own closed loop control algorithms, in essence providing two control loops which counter balance each other. Typically, a cooling control loop based on a liquid coolant operates with a nominal coolant liquid flow (e.g., ˜1 GPM) at all times for the cooling loop to stay at a controlled steady state. As such, coolant liquid in the coolant lines is not allowed to stagnate within the coolant loop.
An effect of this conventional control configuration is that the control effort of each control loop needs to be approximately the same to neutralize an external disturbance quickly, such as an input of waste heat energy from a RF generator driving a plasma. When this external disturbance happens to be large, the control effort to neutralize the disturbance must be made correspondingly large. For example, a heat sink control loop must provide a large sink by operating at a very low temperature and/or having a large thermal mass, etc. However, during times when the external disturbances are much less, for example when a plasma processing system is in an idle state and there is no plasma power input to the system, the cooling effect of the large heat sink cannot be completely removed where a coolant loop maintains a nominal coolant flow. Instead, even during such idle times, the cooling effect is actively countered by the second controller via application of a significant amount of heating energy (e.g., 3000 W, or more) to maintain the setpoint temperature. In addition to this inefficiency, another effect of the conventional control configuration is that the upper limit of the component temperature is limited by the activity of the large heat sink. For example, even with application of 100% heating power, the effect of the large heat sink limits the maximum component temperature to a value less than what would be possible if the heat sink activity could be further reduced. For a similar reason, the transient response to increases in the setpoint temperature is also slow. The end result of the convention configuration is energy inefficient system operation with limited processing temperature range and increased transient response times.